CIRCUIT TO REDUCE DROPOUT VOLTAGE IN LOW DROPOUT VOLTAGE REGULATOR

Abstract: An integrated circuit voltage regulator employs a PNP pass transistor (13) to produce a low dropout voltage. Saturation in the pass transistor (13) produces excessive substrate current which appears in the form of wasted current which lowers the regulator efficiency. A sat catcher circuit is employed to avoid pass transistor (13) saturation. The sat catcher has its operation controlled dynamically so the dropout voltage is minimized and maintains good performance at high regulator output currents.

Description: CIRCUIT TO REDUCE DROPOUT VOLTAGE IN LOW DROPOUT VOLTAGE REGULATOR Background of the Invention In voltage regulators dropout is defined as the input-output voltage differential at which the circuit ceases to regulate against further reductions in input voltage. A low dropout voltage is of maximum interest in battery-operated equipment where the supply voltage declines with time. First, a low dropout voltage means that less power is dissipated in the pass transistor so that efficiency is improved. Second, as the battery voltage declines with time, low dropout voltage means that a greater voltage decline can be tolerated before the battery must be replaced or recharged. In the typical low dropout voltage regulator, using conventional IC construction, the pass transistor is con¬ structed as a large area PNP lateral transistor. Figure 1 is a schematic diagram of a typical low dropout IC voltage regulator. The circuit is typically manufactured using silicon ' epitaxial planar, PN junction isolated, construction which is well known in the art. The circuit receives a + input at terminal 10, referenced against ground terminal 11, and provides a regu¬ lated output at terminal 12. PNP pass transistor 13 has an area that is from 25 to several hundred times that of a minimum area device. The base of transistor 13 is driven by a common emitter NPN driver 14, which has a biasing resistor 15 connected between its base and emitter. This resistor sets the current flowing in transistor 17. Emitter resistor 16 degenerates the gain in transistor 14 and its collector current is set by source 38. Common collector NPN transistor 17 acts as an emitter follower that drives the base of transistor 14 through resistor 18. PNP transistor 19 acts as a bias level shifting emitter follower that drives the base of transistor 17. Current source 20 sets the emitter current in transistor 19. A differential amplifier (diff-amp) 21 forms the amplifier input stage. The current in transistors 22 and 23, which respectively form the noninverting and inverting inputs, are set by the tail current source 24. NPN transistors 25 and 26 form a current mirror load in input stage 21. Load input transistor 25 is diode connected and includes base resistor 27. Load output tran¬ sistor 26 and the output of transistsor 23 provides a single ended drive for the base of transistor 19. Transistor 26 also includes base resistor 28 and a frequency compensation network composed of resistor 29 and capacitor 30. A conventional bandgap reference circuit 31 produces a temperature independent constant voltage which is connected to the base of transistor 22. This reference voltage is typically 1.25 volts. Resistors 32 and 33 form a voltage divider connected between output termnal 12 and ground. The divider tap, node 34, is connected to the base of transistor 23 to provide the regulator negative feedback which stabilizes the circuit operation. The output voltage at terminal 12 will be driven to that level, which results in the voltage at node 34 being equal to the reference voltage at the base of transistor 22. Since a high gain negative feedback loop is involved, the output voltage will be held constant regardless of changes in temperature, input voltage and regulator load current. When a PNP transistor, such as element 13 goes into saturation, its construction is such that it will inject minority carriers into the IC chip N type epitaxial region. These carriers are collected by the P type isolation material and thereby flow into the chip substrate. This substrate current can cause voltage drops along the chip which can adversely affect adjacent active devices. Furthermore, this excessive substrate current is lost and contributes nothing to the output current. Thus, it only serves to heat the IC chip and represents a reduction in efficiency. Accordingly, a circuit action is incorporated into the structure to reduce or avoid saturation in transistor 13. This circuit action is designated a "sat catcher" and is accomplished by transistor 35 which operates in the following manner. PNP transistor 35 has its emitter connected to the collector of transistor 13 and its base is connected to the base of transistor 13. under normal operating conditions sat catcher 35 will be off. As transistor 13 approaches saturation, and its collector rises above its base, sat catcher 35 will turn on and supply current to the base of transistor 36, which will thereby conduct and pull the base of transistor 14 down which will reduce the drive to the base of transistor 13 which rises. When sat catcher 35 is off, during normal circuit operation, resistor 37 returns the base of transistor 36 to ground thereby turning it off. It can be seen that conduction in sat catcher 35 will clamp the collector of transistor 13 at a potential equal to -. _ _V BE35 « This means that the regulator regulator dropout potential is increased from of transis- tor 13 base to emitter potential differential between transistsor 13 and 35 which, while higher than a V , is still well below a V BE. Figure 2 is a graph showing the performance of the figure 1 circuit at 25°C. Curve 39 is a plot of the Vo_h-_- of transistor 13. Curve 40 shows a theoretical linear plot of 60 mv/decade which serves to show the departure of the V βE of transistor 13 from theoretical impurity, at the higher currents. Curve 41 is a plot of the V D lit of transistor 35. The regulator dropout voltage would be curve 41 subtracted from curve 39. Clearly, at high currents, the V R „ of transistor 13 dominates the dropout voltage. Summary of the Invention It is an object of the invention to reduce the dropout voltage in a voltage regulator using a PNP pass tran¬ sistor in which heavy saturation of the output PNP is avoided. It is a further object of the invention to employ a sat catcher in a voltage regulator circuit in which the heavy saturation PNP pass transistor is avoided and the dropout voltage is dynamically decreased as a function of pass transistor current. These and other objects are achieved as follows. A voltage regulator employs sat catcher circuit which avoids heavy saturation in the PNP pass transistor. A small portion of the pass transistor current is mirrored into the sat catcher transistor so that its V rises along with pass transistor current. Accordingly, the dropout voltage does not rise as steeply with current as is the case where the sat catcher current Brief Description bf the Drawing Figure 1 is a schematic diagram of a prior art voltage regulator IC that employs a PNP pass transistor and a sat catcher. Figure 2 is a graph showing the V „ of the PNP pass transistor and the sat catcher of figure 1 as a function of output current. Figure 3 is a schematic diagram of the circuit of the invention. Figure 4 is a graph showing the V βE of the PNP pass transistor and the sat catcher of figure 3 as a function of output current. Figure 5 is a schematic diagram of an alternative circuit employing the invention. Description of the Invention Figure 3 is a schematic diagram of a voltage regulator employing the invention. Where the parts function, as do those of figure 1, the same numerals are employed. All of the components, 10 through 34 and 36 through 38, function as they do those of figure 1. However, the current passed by sat catcher 35 is obtained differently. While in figure 1 the current flowing in sat catcher 35 is essentially constant and equal in value to: J 35 " V BE36 /R 37 where: V 3β is the base to emitter voltage of transistor 36 and R__ is the value of resistor 37. As can be seen, in curve 41 of figure 2, this potential is sub¬ stantially constant. In figure 3, transistor 42 has its base-emitter circuit in parallel with that of PNP pass transistor 13 and will mirror a small fraction of the regulator V terminal 12 current. Therefore, the current flowing into current mirror 41 will vary with regulator load current. Transistor 42 is made to be a small fraction of the size of transistor 13 (a typical ratio is 1/400) so that a small current proportional to output load current will flow into the current mirror 41. The reflected output current flows in diode connected transistor 43 and resistor 45. Under dropout conditions mirror 41, output transistor 44 and resistor 46 will then sink a variable current from sat catcher 35, which no longer operates at a relatively constant current. As PNP pass transistor 13 is pushed closer to saturation to supply increasing output current, the current in sat catcher 35 will now be v BE 3s/ R 3 7 Plus the collector current of transistor 44. Thus, any increase in the V- DC.,i of transistor 13 is partially offset by an increase of the V_ BE_. of sat catcher 35. This action is shown in the graph of figure 4. It can be seen that curve 39 (the V of transistor 13) is the same as that of figure 2, but the V of sat catcher 35, as shown in curve 47, rises propor¬ tionally. This is to be contrasted with curve 41 of figure 2. Since the difference between curves 39 and 47 is substantially reduced at the higher current values, the regulator circuit high current dropout is substantially reduced. Typically, at 400 ma curve 47 of figure 4 will be about 100 mv higher that curve 41 of figure 2. A proportionate reduction in dropout voltage is present. Figure 5 is a schematic diagram of an alterna¬ tive circuit using the invention. Again, where the components operate the same as those of figure 1, the same numbers are used. Here sat catcher 35' is connected differently. Its base is connected to the base of transistor 13 its collector is returned to the collector of transistor 25 and its emitter is returned via a relatively small value (on the order of 200 ohms) resistor 48 to the collecxtor or transistor 13. The collector of transistor 44 is connected to the juncture of the emitter of sat catcher 35' and resistor 48. When the PNP pass transistor 13 approaches saturation, sat catcher 35' will turn on and inject current into the collector of transistor 25. This injected current will offset the error amplifier in such a way as to reduce the base drive to the pass PNP transistor 13. It can be seen that the collector current of transistor 44, which tracks the regulator load current, flows in resistor 48, thereby producing a voltage drop which will add to the V a_ t_o of of the sat catcher 35'. In this embodiment the V D _ of sat catcher 35' remains relatively constant and the voltage drop across resistor 48 provides the dynamic dropout reduction. Example The circuit of figure 5 was constructed using conventional monolithic silicon IC construction with planar, epitaxial, pn junction isolated parts. PNP pass transistor 13 had an area of about 400 times that of transistor 42 so that at an output of 150 ma, the current in transistor 42 was about 0.4 ma. The following components were employed: COMPONENT VALUE Resistor 16 18 ohms Resistor 18 0 ohms Current Source 20 3 microamperes Current Source 24 6 microamperes Resistor 27 110 ohms Resistor 28 100 ohms Resistor 29 350 ohms Capacitor 30 40 pf Current Source 38 3 microamperes Resistor 32 135.7k ohms Resistor 33 42.9k ohms Resistor 45 1.0k ohms Resistor 46 2.0k ohms Resistor 48 400 ohms In place of resistor 15, an 0.06uA current source was used from the base of transistor 14 to ground. The circuit produced a regulated output of 5 volts and could supply over 150 ma without saturating transistor 13. The maximum dropout voltage at 150 ma was 250 ma millivolts. With transistor 44 disabled, the dropout was lOOmv higher. The invention has been described and a preferred embodiment detailed. Alternatives have also been described. When a person skilled in the art reads the foregoing descrip¬ tion, other alternatives and equivalents, within the spirit and intent of the invention, will be apparent. Accordingly, it is intended that the scope of the invention be limited only by the claims that follow.

Claims: I cla im : 1. In an integrated voltage regulator circuit employing a PNP pass transistor means for preventing pass tran¬ sistor saturation comprising: a PNP sat catcher transistor having an emitter coupled to said pass transistor collector, a base coupled to said pass transistor base and a collector; and means coupled to said sat catcher transistor for varying the current flowing therein in proportion to the current flowing in said pass transistor whereby the base to emitter voltage of said sat catcher transistor rises with an increase in pass transistor current. 2. The integrated voltage regulator of claim 1 wherein said means coupled to said set catcher transistsor comprise: a PNP current source transistor having its emitter-base electrodes connected in parallel with those of said pass transistor whereby a sense current is sourced from the collector of said current source transistor; and an NPN current mirror having an input coupled to said collector of said current source~transistor whereby said NPN current mirror sinks the current source by said source transistor, and an output that sinks a current proportional to the current sunk at said input, said output being connected to said collector of said sat catcher transistor. 3. The integrated voltage regulator circuit of claim 2 wherein said pass transistor is much larger in area than said current source transistor whereby said sense current is a snail fraction of the regulator current flowing in said pass transistor. 4. The integated voltage regulator circuit of claim 3 wherein a resistor is coupled in series with said sat catcher emitter and said NPN current mirror output is connected to the emitter of said sat catcher transistor.

Inventors: CECIL JAMES B, CECIL, JAMES, B.
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1994
Main classification: G05F3
IPC codes: G05F3